Swedish is fine for me.
So just to summon up :
a) One card each for long term tickets (24/72 hours, 7 days).
b) Possibility of sharing a card ( with a bit of trouble ) if you use the card to load money on ( as rejsekasse ) and only buy normal tickets ( single zone tickets ).
c) 24/72 hours tickets can be bought without having the access plastic card
(issued as a one time use paper/ cardboard ticket with a chip).
I always find it to be much cheaper and much more convenient to have a long term ticket in Stockholm.